(478a) The Synergy between Experiment and Theory in Catalysis Research

Recent advances in theoretical methods and accelerated computing powers have open doors to easily access and use quantum mechanical calculations in researching heterogeneous catalysis. Such methods, in turn, have enabled to understand the nature of active centers in solid catalysts and the detailed reaction mechanisms, which were often difficult to assess solely from experiments. Indeed, the development of density functional theory (DFT) methods have allowed to make a breakthrough in catalysis research, and now, it is a common practice in the field to collaborate between experiment and theory to solve difficult problems. Yet, rigorously connecting experiment and theory has remained as a challenge, including a strategy to find appropriate catalyst models that accurately capture the nature of active sites in working conditions. In this work, we demonstrate how to rigorously combine kinetic, isotopic, spectroscopic, and transient experiments with DFT calculations to reveal the mechanistic details of HCOOH dehydration on TiO₂ catalysts and their acid-base properties that determine their reactivities.

In-situ infrared spectra, combined with transient experiments, demonstrate that all Ti_5c-O_2c acid-base site pairs in TiO₂ samples are saturated with bidentate formates (*HCOO*) during catalytic turnovers (0.1-4 kPa HCOOH; < 500 K), requiring theory to model TiO₂ surfaces fully saturated *HCOO* species. In doing so, we show that HCOOH adsorbs molecularly (HCOOH*) on top of the *HCOO* adlayer to form a second adlayer through interactions of HCOOH* with the H-atom in the *HCOO* adlayer. These HCOOH* species undergo dehydration via a transition state that is stabilized by interactions with a Ti_5c center, which is made available only by the momentary formation of HCOOH* from a vicinal *HCOO*. Such mechanistic interpretation agrees well with dehydration kinetics and kinetic isotopic effects and explains the 10-times lower dehydration rates on rutile TiO₂ than of anatase TiO₂; stronger Ti_5c acid centers in rutile bind *HCOO* species much more strongly, requiring larger energy to “shove” such *HCOO* species for HCOOH* dehydration. The results of this work, in turn, illustrate how diverse experimental tools and theoretical methods must collaborate to provide definitive mechanistic conclusions and how these methods must accept and rigorously consider the crowded nature of surfaces during practical catalysis.